We are utilizing the chinese hamster ovary (CHO) fibroblast grown in vitro to study the genetics and biochemistry of some aspects of the behavior of cultured cells. Our work has emphasized cell adhesion, morphology, response to cyclic AMP, the role of cell surface antigens, receptors and internalization of their ligands in cell behavior, and the mechanism of cell transformation by tumor viruses. We have isolated a variety of different mutants with abnormal adhesion, abnormal morphology or abnormal response to cyclic AMP and some of these have been characterized as to their biochemical defects. We are developing a system for isolating cell mutants resistant to transformation by the product of the src gene of Rous sarcoma virus. We are using specific antisera and complement to isolate CHO mutants which have lost specific cell surface antigens or are unable to secrete specific antigens. We are establishing general procedures for the isolation of mutants lacking specific cell surface receptors, and mutants unable to internalize specific ligands.